Pressurized water nuclear reactors contain a core consisting of assemblies in which the neutron reactions giving rise to the power released by the core take place. In order to control this power, in certain core assemblies control rods of neutron-absorbing material are displaced in the vertical direction over the entire height of the assemblies. The control rods formed by bundles of absorbing rods of great length are guided by members extending in line with the guiding tubes of the assemblies into which the control rods are introduced, and disposed above the core. These members are disposed inside guiding tubes forming part of the upper internal equipment of the nuclear reactor. These guiding tubes are fixed to the top core plate with the aid of pins provided with a screw thread, onto which is screwed a nut which comes to bear against the foot of the guiding tube. The pin is in turn fixed inside the top core plate by its bottom part.
In the operation of pressurized water nuclear reactors, it has been found that the mechanical properties of the pins fastening the feet of guiding tubes of the upper internal equipment may deteriorate after a certain period of operation of the reactor. In some cases, fractures of these fastening pins could even be observed. It was possible to attribute these defects to inadequate tightening of the screwed connection between the fastening pins and the foot of the guiding tube. If the tightening is too slack, the guiding tube is not perfectly immobilized, and this defect increases during the operation of the reactor, in the course of which the guiding tubes are subjected to the circulation of pressurized water at high temperature and with a very high rate of flow.
In cases where the tightening of the pins is excessive, the tensile stresses in the pin give rise to stress corrosion of the latter.
In the first case, fatigue fractures occur, and in the second case fractures are caused by cracking due to corrosion.
For the purpose of mounting the fastening pins on the feet of the guiding tubes, the nuts are tightened on the pins by hand with the aid of a torque wrench. The accuracy achieved in respect of tightening torque is insufficient to enable the tightening defects described above to be avoided.